(a) Technical Field
The present disclosure relates to a power configuration system for a fuel cell hybrid vehicle and a method for controlling the same. More particularly, the present invention relates to a power configuration system for a fuel cell hybrid vehicle and a method for controlling the same, which maintains the voltage of a fuel cell below that of a storage means during regenerative braking so that the fuel cell may not unnecessarily charge the storage means, thereby preferably increasing the amount of regenerative braking energy and suitably improving fuel efficiency.
(b) Background Art
A fuel cell is an electricity generation system that does not convert chemical energy of fuel into heat by combustion, but electrochemically converts the chemical energy directly into electrical energy in a fuel cell stack. Preferably, the fuel cell can be applied to the electric power supply to small-sized electrical and electronic devices, particularly portable devices, as well as industrial and household appliances and vehicles.
Among the most attractive fuel cells for a vehicle is a proton exchange membrane fuel cell or a polymer electrolyte membrane fuel cell (PEMFC). A PEMFC includes a fuel cell stack comprising a membrane electrode assembly (MEA), a gas diffusion layer (GDL), a gasket, a sealing member, and a bipolar plate. The MEA includes a polymer electrolyte membrane through which hydrogen ions are transported. An electrode/catalyst layer, in which an electrochemical reaction takes place, is suitably disposed on each of both sides of the polymer electrolyte membrane. The GDL suitably functions to uniformly diffuse reactant gases and transmit generated electricity. The gasket suitably functions to provide an appropriate airtightness to reactant gases and coolant. The sealing member suitably functions to provide an appropriate bonding pressure. The bipolar plate suitably functions to support the MEA and GDL, collect and transmit generated electricity, transmit reactant gases, transmit and remove reaction products, and transmit coolant to remove reaction heat, etc.
Preferably, the fuel cell stack is composed of a plurality of unit cells, each of the unit cells including an anode, a cathode, and an electrolyte (electrolyte membrane). Hydrogen as fuel is suitably supplied to the anode (“fuel electrode”, “hydrogen electrode”, or “oxidation electrode”) and oxygen as oxidant is suitably supplied to the cathode (“air electrode”, “oxygen electrode” or “reduction electrode”).
The hydrogen supplied to the anode is dissociated into hydrogen ions (protons, H+) and electrons (e−) by a catalyst that is preferably disposed in the electrode/catalyst layer. The hydrogen ions are suitably transmitted to the cathode through the electrolyte membrane, which is a cation exchange membrane, and the electrons are suitably transmitted to the cathode through the GDL and the bipolar plate.
At the cathode, the hydrogen ions supplied through the (polymer) electrolyte membrane and the electrons transmitted through the bipolar plate react with the oxygen in the air supplied to the cathode to produce water. Migration of the hydrogen ions causes electrons to flow through an external conducting wire, which generates electricity and heat.
Preferably, fuel cell hybrid vehicles including large vehicles, such as buses, as well as small vehicles, have a system equipped with an electricity storage means such as a high voltage battery or a supercapacitor as an auxiliary power source for suitably providing the power required to drive a motor in addition to the fuel cell as a main power source.
The present invention features a fuel cell-supercapacitor hybrid vehicle that does not employ a power converter. The fuel cell-supercapacitor hybrid vehicle has certain preferred advantages, for example, but not limited to, high fuel efficiency (high regenerative braking, high efficiency of supercapacitor, and without the use of the power converter), an increase in durability of the fuel cell, high reliability control (automatic power assist and automatic regenerative braking function).
For example, in the hybrid vehicle in which the fuel cell and the supercapacitor are suitably directly connected, the fuel cell continuously outputs power at a constant level during driving. Accordingly, if there is surplus power, the supercapacitor is suitably charged with the surplus power, whereas, if there is insufficient power, the supercapacitor suitably supplies the insufficient power to drive the vehicle.
A power configuration system of a fuel cell-supercapacitor hybrid vehicle as set forth by preferred embodiments of the present invention is described below.
FIG. 1 is a schematic diagram showing an exemplary power configuration system of a conventional fuel cell-supercapacitor hybrid vehicle. Preferably, the power configuration system of the fuel cell-supercapacitor hybrid vehicle includes a fuel cell 10 suitably used as a main power source, a supercapacitor 20 suitably used as an auxiliary power source, and a motor control unit (MCU) 40, which preferably includes an inverter and is a power module that operates a drive motor 41. The MCU 40 is suitably connected to output terminals of the fuel cell 10 and the supercapacitor 20 to produce 3-phase pulse width modulation (PWM) by receiving direct current from the fuel cell 10 and the supercapacitor 20 and to control the operation of the drive motor 41 and regenerative braking.
Preferably, in the above power configuration system, the fuel cell 10, which receives hydrogen from a hydrogen tank and air from an air blower and generates electricity by an electrochemical reaction between hydrogen and oxygen in the air, is suitably used as the main power source. In certain embodiments, the drive motor 41 and the MCU 40 are directly connected to the fuel cell 10 through a main bus terminal 30, and the supercapacitor 20 is connected to the main bus terminal 30 to provide power assist and regenerative braking.
Preferably, a low voltage DCDC converter (LDC) 50 for voltage conversion and an auxiliary battery (e.g., 12V auxiliary battery) 51 for driving auxiliary components are connected to the main bus terminal 30. Moreover, balance of plant (BOP) components for driving the fuel cell 10, such as an air blower 11, a water pump 12, a radiator fan 13, and a hydrogen recirculation blower 14, are suitably connected to the main bus terminal 30 through a high voltage junction box 15 to facilitate the starting of the fuel cell 10.
Furthermore, a reverse blocking diode 31 for preventing reverse flow of current is installed in the main bus terminal 30, and a load device for preventing voltage generation while the operation of the fuel cell 10 is stopped is connected to the fuel cell 10. As the load device, a heater resistor (COD) 16 may be connected to the output terminal of the fuel cell 10 through a high voltage junction box 17. The heater resistor 16 consumes the power of the fuel cell 10 during initial start-up to rapidly heat coolant of the fuel cell stack (temporarily used during initial start-up).
In the above-described power configuration system of FIG. 1, the fuel cell 10 and the supercapacitor 20 are suitably connected in parallel. During initial start-up, the power of the auxiliary battery 51 is boosted to a high voltage by the LDC 50 and then supplied to the high voltage components 11 to 14 through the high voltage junction box 15, thus driving the fuel cell 10. When the start-up of the fuel cell 10 is completed, the LDC 50 enters a 12 V charging mode, and the supercapacitor 20 starts charging and, when the charging is completed, suitably drives the vehicle with discharge power.
Driving modes of the hybrid vehicle, preferably equipped with the fuel cell as the main power source and the supercapacitor as the auxiliary power source, includes an electric vehicle (EV) mode in which the motor is suitably driven only by the power of the fuel cell, a hybrid electric vehicle (HEV) mode in which the motor is suitably driven by the power of the fuel cell and the power of the supercapacitor at the same time, and a regenerative braking (RB) mode in which the supercapacitor is suitably charged.
However, the fuel cell-supercapacitor hybrid vehicle has a consideration in that the supercapacitor is automatically charged by the fuel cell, which restricts the regenerative braking. During braking of the vehicle, a considerable amount of regenerative braking energy generated in the drive motor is stored in the supercapacitor. In this case, since the load of the fuel cell is removed, the voltage of the fuel cell is suitably increased to charge the supercapacitor.
When the amount of electrical energy stored in the supercapacitor is smaller, it is possible to store a larger amount of regenerative braking energy provided by the drive motor in the supercapacitor. Accordingly, in order to store a greater amount of regenerative braking energy in the supercapacitor, it is necessary to reduce the amount of electrical energy charged by the fuel cell in the supercapacitor at least during the regenerative braking, and it is thus possible to prevent deterioration of fuel efficiency.
As such, during the regenerative braking, the fuel cell should not charge the supercapacitor with the electrical energy thereof and, accordingly, the voltage of the fuel cell should be lower than that of the supercapacitor. However, as the supercapacitor is suitably charged during braking of the vehicle, the voltage of the fuel cell is gradually increased to reach an open circuit voltage (OCV) value. And, accordingly, the number of times each of the unit cells of the fuel cell reaches a predetermined voltage (e.g., 0.85 V) is increased, and thus the durability of the fuel cell is considerably reduced. In order to improve the durability of the fuel cell, the voltage per cell of the fuel cell should preferably be maintained below a predetermined level (e.g., 0.85 V).
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.